1. Field of the Invention
The present invention relates to transducers of the half-wave type using as active element a piezoelectric polymer foil such as vinylidene polyfluoride.
2. Description of the Prior Art
These transducers have interesting applications in the field of medical diagnosis by examination of tomoechographic images. Piezoelectric polymer materials offer the advantage of having an acoustic impedance of the same order of size as that of the propagation media. These materials are also easy to use, for they may be molded, thermoformed and their flexibility may be turned to account. On the other hand, these materials have relatively low piezoelectric properties compared for example with those of piezoelectric ceramics and they pose technological problems particularly in so far as the mediocre adherence of the metal layers intended for forming the electrodes of a transducer is concerned. When aluminium, nickel, chromium or gold is deposited by evaporation in a vacuum on a polyvinylidene fluoride foil, difficulties are met with for providing certain electrode configurations somewhat extensive or corresponding to a complicated pattern. When a very high acoustic impedance reflecting medium is available contiguous to the rear face of a piezoelectric polymer foil, the electrode deposits may be effected on this medium provided that it is insulating and that it allows a better adherence. However, the transducers formed according to this principle operate at a quarter wave and it has been discovered that the conversion losses, the depth resolution and the pass-band are not as good as with a configuration operating at half-wave or full-wave. Half-wave operation in fact allows a closer approach to a perfect reflection of the waves towards the external face of the transducer, for the medium loading the rear face of the transducer has an acoustic impedance typically equal to that of air, i.e. practically zero. On the other hand, such a non dissipating medium does not form an appropriate support for depositing electrodes thereon and another means must be used to solve the problem posed, i.e. the lack of adherence of metal deposits to piezoelectric polymers.
Configurations of the half-wave or full-wave type perform better in so far as the pass-band and sensitivity are concerned. They are usually formed by a piezoelectric polymer foil whose external face is coupled to the biological medium via an impedance matching quarter-wave layer and whose internal face is directly in relation with the air contained inside a case or with a similar low impedance medium, a polymer foam for example. The distance between external and internal faces is wholly occupied by the active material whose thickness corresponds very often to a half wavelength of the central operating frequency.
While maintaining the principle of half-wave or full-wave excitation, the invention suggests forming the vibrating structure by bringing together several layers one of which, the active one, provides the transducer effect, the others simply playing the role of electrode supports. These electrode carrying layers are integral with the active layer so that a stratified vibrating structure is obtained which remains half-wave as a whole although partially active. Since the layers fixed by bonding to the active layer may have their electrodes in contact with the active layer, the problem of adherence is solved by maintaining a small spacing between the electrodes and perfect reflection of the waves at the rear face of the composite structure, which is in direct relation with a very low acoustic impedance reflecting medium.